What did British officers think of the American civil war as it was happening?

without facts, I would expect some central Asian horse army to have 200,000 humans, far earlier than that.. Persian and Egyptian armies were large at different times too.. sounds uninformed..

No OP's correct, the US surpassed the UK both in population and in GDP sometime around the middle of the 19th century.

My bad, just got back from a run and obviously my brain was scrambled.

The Civil War in America absolutely dominated the British national conversation, especially in the early years of the war. Politicians followed the conflict closely, and there were even several debates in Parliament over British policy toward the conflict.

Frequently discussed was a line that a lot of Americans would recognize today, of "when should Britain get involved??" because of the destructiveness of the conflict. Prime Minister Palmerston & Foreign Secretary Russell spent a lot of time maneuvering and deflecting calls for Britain to get involved or pick a side.

A few other bits I found surprising when studying this topic—

  1. Some British MPs were very pro-Confederate, and pushed for recognition of the Confederacy as a real country in Parliament.
  2. British war correspondents were on the ground with both Union and Confederate armies, and sent regular dispatches to British newspapers.
  3. British (and other European) officers regularly volunteered on *both* sides.
  4. It was fashionable for a time in Britain to be pro-Confederate. Confederate propagandist networks *in* Britain brilliantly played down slavery and played up "self-determination".
  5. Britain nearly declared war on the Union (Trent affair), to the point that Royal Navy was just waiting for the go-signal to commence hostilities & Britain sent thousands of additional troops to Canada.
  6. There were *tons* of ironclads already in European fleets, there had just never been a fight between ironclads! Europeans watched the Monitor v. Merrimack battle & adapted their fleets & battle doctrines accordingly.
  7. The British cabinet had a very serious, "can we even win a war against the U.S. anymore?" conversation at the end of the conflict, after witnessing the million-man army of the Union, the Richmond campaign, and growing effectiveness of the U.S. Navy.
  8. Americans credit Seward as a brilliant Secretary of State during the conflict, but in Britain and France he was considered foolish, dangerous, and unpredictable—a lot of the tension between the Union and Europe can be laid at his feet.
  9. Prussian military observers watched how the Union used railroads to move massive numbers of troops & supplies around, and adopted a lot of the Union tactics to absolutely crush the French just a few years later. (Franco-Prussian war of 1870.) Seriously - there were European observers *all over the place*.

Strongly recommend A World on Fire: Britain's Crucial Role in the American Civil War by Amanda Foreman. Brilliant book, and a page-turner.

Militarily Europe walked away with simple caricatures of the war, but the lionization of Ulysses S. Grant. Grant was (rightly) recognized as a new kind of general - one that would be far more familiar in World War I than in the Napoleonic War. This led to a lot of fame.

Generally, the US revolutionary war was viewed very positively in parts of Europe. That upstart new world was finally in its place and would now tear itself to pieces without European governance. Britain considered entering the war early on(on the southern side!) and benefited from the economic collapse of the Americas. The British commercial fleets went from near-parity with the US merchant marine to unquestioned ruler of the commercial seas by the devastation of the blockades and the lack of insurance underwriting for north or south flagged vehicles.

But a lot of things were missed millitary that they would learn in 1914. The role of rail lines and junctions, and the early signs of trench warfare and total war were the only thing that could break near-industrial societies. The potential of armored battleships and submarines. (Though to be fair, imagine how much history would have changed if the Turtle had detonated its torpedo in the Revolutionary War!) These lessons would be learned a hard way sixty years later.